Oligomerization of alkenes in a plurality of successive heterogeneous catalyst zones

ABSTRACT

Process for oligomerizing an alkene stream over a solid catalyst comprising sulfur and nickel, in which the oligomerization is carried out in two or more successive catalyst zones and the molar ratio of sulfur to nickel in the first catalyst zone is less than 0.5 and that in the last catalyst zone is 0.5 or more and, in the case of further catalyst zones between the first and last catalyst zones, the molar ratio of sulfur to nickel in each catalyst zone is not less than that in the immediately preceding catalyst zone, based on the main flow direction of the feed stream.

RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. 371)of PCT/EP2003/006838 filed Jun. 27, 2003 which claims benefit to Germanapplication Serial No. 102 29 763.0 filed Jul. 3, 2002.

The present invention relates to a process for oligomerizing an alkenestream over a solid catalyst comprising sulfur and nickel.

Alkenes having from 2 to 6 carbon atoms and mixtures thereof, inparticular alkenes having 4 carbon atoms, are available in largequantities both from FCC plants and from steam crackers. The C₄fraction, i.e. the mixture of butenes and butanes, obtained in each caseis, after the isobutene has been separated off, particularly well suitedto the preparation of oligomers, in particular octenes and dodecenes.Both the octenes and the dodecenes are, after hydroformylation andsubsequent hydrogenation to form the corresponding alcohols, used, forexample, for the preparation of plasticizers or surfactant alcohols.

In the case of plasticizer alcohols, the degree of branching generallyhas an effect on the properties of the plasticizer. The degree ofbranching is described by the iso index, which indicates the mean numberof methyl branches in the respective fraction. Thus, for example,n-octenes contribute 0, methylheptenes contribute 1 and dimethylhexenescontribute 2 to the iso index of a C₈ fraction. The lower the iso index,the more linear are the molecules in the respective fraction. The higherthe linearity, i.e. the lower the iso index, the higher the yields inthe hydroformylation and the better the properties of the plasticizerprepared therewith. A lower iso index leads, for example, in the case ofphthalate plasticizers, to a reduced volatility and in the case offlexible PVC formulations containing these plasticizers, to improvedcold fracture behavior.

The conversion of alkenes into low molecular weight oligomers overnickel- and sulfur-containing heterogeneous catalysts is prior art.Catalysts suitable for this purpose are described below.

However, the previously known processes of this type have thedisadvantage that because of the decreasing alkene content of the feedstream in the direction of the reactor outlet, a satisfactory conversionof the alkenes into relatively unbranched alkenes has hitherto generallyonly been possible if the temperature is increased in the outlet end ofthe catalyst bed or if a more active catalyst is used in this region orthe total catalyst volume is increased.

It is an object of the present invention to remedy these disadvantagesof the prior art by means of an improved catalyst bed.

We have found that this object is achieved by a process foroligomerizing an alkene stream over a solid catalyst comprising sulfurand nickel, wherein the oligomerization is carried out in two or moresuccessive catalyst zones and the molar ratio of sulfur to nickel in thefirst catalyst zone is less than 0.5 and that in the last catalyst zoneis 0.5 or more and, in the case of further catalyst zones between thefirst and last catalyst zones, the molar ratio of sulfur to nickel ineach catalyst zone is not less than that in the immediately precedingcatalyst zone, based on the main flow direction of the feed stream.

For the purposes of the present invention, the term “oligomers” refersto dimers, trimers and higher oligomers of the alkenes. The process ofthe present invention is particularly useful for preparing dimers ofthese alkenes.

As starting materials, preference is given to using alkenes having from2 to 6 carbon atoms, mixtures of such alkenes with one another ormixtures of such alkenes with alkanes. The oligomerization process ofthe present invention is particularly suitable for the reaction ofmixtures of alkenes having 3 and in particular 4 carbon atoms,especially hydrocarbon streams which comprise 1-butene and/or 2-buteneand one or more butanes and are essentially free of isobutene.

The reactor is generally a (cylindrical) tube reactor. As analternative, the reactor used can be a reactor cascade comprising aplurality of, preferably two or three, such tube reactors (subreactors)connected in series, as is known, for example, from WO-A 99/25668 orWO-A 01/72670.

The catalysts over which the oligomerization is carried out areheterogeneous catalysts comprising sulfur and nickel. They are generallyknown, for example from FR-A 2 641 477, EP-A 272 970, WO-A 95/14647 andWO-A 01/37989, U.S. Pat. No. 2,794,842, U.S. Pat. No. 3,959,400, U.S.Pat. No. 4,511,750 and U.S. Pat. No. 5,883,036, which are hereby fullyincorporated by reference in respect of the sulfur- andnickel-containing catalysts disclosed therein. Particularly usefulcatalysts of this type are those which lead to a low degree of branchingin the oligomers obtainable therewith, especially those described inWO-A 95/14647, WO-A 01/37989 and the documents of the prior art cited inthis context in these two documents.

The totality of the catalyst over which the starting material passes inthe reactor will hereinafter be referred to as the fixed catalyst bed.If a reactor cascade is employed, the fixed catalyst bed is generallydistributed over all the subreactors of the cascade.

The total fixed catalyst bed is, according to the present invention,divided into two or more successive catalyst zones. In this context, acatalyst zone is a section of the fixed catalyst bed in the flowdirection of the feed. Such a catalyst zone has a specificsulfur-to-nickel ratio compared to the adjacent catalyst zone(s),especially, when the catalyst zone is located between two other catalystzones, these two adjacent catalyst zones. In the case of a reactorcascade, a catalyst zone can be located within a single subreactor orcan be distributed continuously over two or more successive subreactors,without the fixed catalyst bed in the first and last subreactor havingto be made up entirely of this catalyst zone.

The catalysts of the first and last catalyst zones in particular caneach be accommodated in a single reactor or in a cascade of reactors forthe purposes of the reaction.

The feed stream can also be divided and the substreams obtained in thisway can be fed to the fixed catalyst bed at different places, forinstance when using a reactor cascade at the points between theindividual reactors. It is also possible for a substream of the totalfeed stream to be fed in before the beginning of a catalyst zone or,particularly when a catalyst zone extends from one subreactor to thenext in a cascade, at the point at which the catalyst zone is dividedbetween the two subreactors.

As catalysts for the first catalyst zone, in which the molar ratio ofsulfur to nickel is less than 0.5, preference is given to usingcatalysts of this type as described in WO-A 95/14647 or WO-A 01/37989.

As catalysts for the last catalyst zone, in which the molar ratio ofsulfur to nickel is 0.5 or more, preference is given to using catalystsof this type as described in FR-A 2 641 477, EP-A 272 970, U.S. Pat. No.3,959,400 or U.S. Pat. No. 4,511,750, in particular those having a molarratio of sulfur to nickel of more than 0.8 and very particularlypreferably those having a molar ratio of sulfur to nickel of greaterthan or equal to 1.

The process of the present invention is preferably carried out over afixed catalyst bed in which the molar ratio of sulfur to nickel in thefirst catalyst zone is less than 0.4 and that in the last catalyst zoneis more than 0.6.

In a further, preferred embodiment, the process of the present inventionis carried out over a fixed catalyst bed in which the molar ratio ofsulfur to nickel in the first catalyst zone is less than 0.4 and that inthe last catalyst zone is more than 0.8.

In a further, preferred embodiment, the process of the present inventionis carried out over a fixed catalyst bed in which the molar ratio ofsulfur to nickel in the first catalyst zone is less than 0.4 and that inthe last catalyst zone is equal to or more than 1.

The second and all further catalyst zones of the fixed catalyst bed willhereinafter be referred to as “remaining catalyst zones” to distinguishthem from the first catalyst zone.

The process of the present invention is preferably carried out in such away that the alkenes of the feed stream are reacted to an extent of from50 to 99%, preferably from 65 to 99%, especially from 80 to 99% and inparticular from 90 to 99%, in the first catalyst zone in which the molarratio of sulfur to nickel is less than 0.5.

Furthermore, the process of the present invention is preferably carriedout in such a way that the alkenes of the feed stream are reacted to anextent of from 10 to 99%, preferably from 50 to 99%, in the remainingcatalyst zones.

In a further, preferred embodiment, the process of the present inventionis carried out in such a way that the alkenes in the feed stream arereacted to an extent of from 50 to 99% in the first catalyst zone andthe alkenes remaining unreacted after this first catalyst zone arereacted to an extent of from 10 to 99% in the remaining catalyst zones.

In a further, preferred embodiment, the process of the present inventionis carried out in such a way that the alkenes in the feed stream arereacted to an extent of from 65 to 99% in the first catalyst zone andthe alkenes remaining unreacted after this first catalyst zone arereacted to an extent of from 10 to 99% in the remaining catalyst zones.

In a further, preferred embodiment, the process of the present inventionis carried out in such a way that the alkenes in the feed stream arereacted to an extent of from 80 to 95% in the first catalyst zone andthe alkenes remaining unreacted after this first catalyst zone arereacted to an extent of from 10 to 99% in the remaining catalyst zones.

In a further, preferred embodiment, the process of the present inventionis carried out in such a way that the alkenes in the feed stream arereacted to an extent of from 80 to 95% in the first catalyst zone andthe alkenes remaining unreacted after this first catalyst zone arereacted to an extent of from 50 to 99% in the remaining catalyst zones.

Furthermore, the process of the present invention is preferably carriedout in such a way that a total conversion of the alkenes in the feedstream of more than 91%, preferably more than 95% and in particular morethan 97%, is achieved over all catalyst zones.

The oligomerization reaction is generally carried out at from 30 to 280°C., preferably from 30 to 190° C. and in particular from 40 to 130° C.,and a pressure of generally from 1 to 300 bar, preferably from 5 to 100bar and in particular from 10 to 50 bar. The pressure is advantageouslyselected so that the feed is supercritical and in particular liquid atthe temperature set. Different reaction conditions in respect ofpressure and/or temperature within these pressure and temperature rangescan be set in the individual tube reactors of a reactor cascade.

The oligomerization process of the present invention can be carried outadiabatically or isothermally.

Otherwise, the way of carrying out the process is sufficiently wellknown to a person skilled in the art, especially from WO-A 99/25668 andWO-A 01/72670, whose relevant contents are hereby fully incorporated byreference.

After leaving the reactor, the oligomers formed are separated in amanner known per se from the unreacted hydrocarbons and the latter are,if desired, returned to the process (cf., for example, WO-A 95/14647).The separation is generally carried out by fractional distillation.

Compared to known processes of this type, the process of the presentinvention leads to a high alkene conversion combined with a low degreeof branching of the oligomers obtainable in this way. This effect hashitherto generally been able to be achieved only by increasing thetemperature in the later part of the catalyst bed or by using a moreactive catalyst in this region or by means of an increased total volumeof catalyst because of the decreasing alkene content of the gas streamin the direction of the reactor outlet.

EXAMPLES

I. Catalysts

The Ni(NO₃)₂.6 H₂O used came from Fluka.

Catalyst “1a” (S:Ni ratio=0)

As described in DE-A 43 39 713, Example 1, a sulfur-free catalyst wasprepared from 50% by weight of NiO, 37% by weight of SiO₂ and 13% byweight of TiO₂.

Catalyst “1b” (S:Ni ratio=0.34)

γ-aluminum oxide of the grade “D10-10” from BASF AG (3 mm starextrudates, BET surface area: 202 m²/g, water absorption capacity: 0.76ml/g, loss on ignition: 1.6% by weight) was used as support.

200 g of this support were impregnated at room temperature with 125 mlof a solution of 125 mmol of 96% strength H₂SO₄ and 361 mmol of 97%strength Ni(NO₃)₂.6 H₂O in water while stirring. The catalyst obtainedin this way was dried in air at 120° C. for 10 hours and calcined in airat 500° C. for 2 hours. The proportion of nickel (“Ni”) was thendetermined as 9.04% by weight and that of sulfur (“S”) was determined as1.67% by weight, in each case based on the total weight of the catalystobtained, and the molar ratio of sulfur to nickel (“S:Ni”) in thecatalyst was determined as 0.34.

The sulfur content of the finished catalyst was determined byquantitative infrared analysis of the sulfur dioxide formed oncombustion of the catalyst. The nickel content could be obtained byICP-mass spectrometry.

Catalyst “1c” (S:Ni ratio=1)

γ-aluminum oxide of the grade “D10-10” from BASF AG (4 mm extrudates,BET surface area: 210 m²/g, water absorption capacity: 0.73 ml/g, losson ignition: 1.8% by weight) was used as support.

400 g of this support were impregnated at room temperature with asolution of 184 g of NiSO₄.6 H₂O in water while stirring. The volume ofthe water used was chosen in accordance with the water absorptioncapacity of the support. The catalyst obtained in this way was dried inair at 120° C. for 16 hours and calcined in air at 500° C. for 2 hours.The proportion of nickel (“Ni”) was then determined as 7.9% by weightand that of sulfur (“S”) was determined as 4.32% by weight, in each casebased on the total weight of the catalyst obtained, and the molar ratioof sulfur to nickel (“S:Ni”) in the catalyst was determined as 1.

II. Oligomerizations

A) Apparatus

FIG. 1 schematically shows an apparatus in which the process of thepresent invention was, by way of example, carried out continuously at 30bar. The alkene-containing stream (hereinafter referred to as the“Feed”) was fed via F into the adiabatic subreactor R1 and from thereconveyed via intermediate cooling ZK to the adiabatic subreactor R2. Thesubreactors had a length of 4 m and a diameter of 0.08 m, so that theyeach had a volume of 20 liters; when only 20 liters of catalyst wereused, all the catalyst was placed in one of the two subreactors, whilethe second subreactor contained steatite spheres as inert material. Theoutput from the reactor R2 was worked up by distillation in the column Kand the oligomeric product was taken off as bottoms via B. The streamtaken from the top of the column K was partly recirculated via Z to thereactor R1, and the other part of this stream was discharged from theapparatus via P (as purge stream).

The reaction pressure, which was higher than the pressure at which theraffinate II was supplied, was generated by means of an upstream reactorfeed pump and was regulated by means of customary pressure maintenancedevices downstream of the reactor.

The butene/butane mixtures shown in table 1 were used.

TABLE 1 (FIGURES are in % by weight based on the total feed stream F)Mixture n-butenes butanes isobutene A 78 19.7 2.3 B 54.2 44.8 1 C 56.942.3 0.8 D 55.8 44.5 0.7 E 26.8 72.9 0.3 F 26.4 73.2 0.4

B) Experimental Procedure

B.1) Reactions Over a Single Catalyst Zone

The butene/butane mixtures shown in table 1 were passed at the meantemperature T over the volume Vol of the catalyst Cat located in theapparatus shown in FIG. 1. The output from the reactor was isolated andanalyzed. Further details of the experimental parameters and the resultsof the experiments are shown in table 2.

B.2) Conversion of the Results from B.1 to 2 and 3 Catalyst Zones

The results from section B.1 were summarized mathematically. Table 3shows the results obtained in this way for the sequences according tothe present invention (No. 1, 4 and 5) of catalyst zones and for thesequences not according to the present invention (No. 2, 3, 6 and 7)determined for comparative purposes.

It can be seen from the experimental results that combination of two orthree catalyst zones to form a fixed catalyst bed in accordance with thepresent invention gives, at comparable alkene conversions and isoindices of the octenes, a space-time yield in respect of octenes anddodecenes which is from 15 to 33% higher than that obtained in thereaction over catalyst beds which are not in accordance with the presentinvention. In addition, these results can be obtained at lower reactiontemperatures over the catalyst, which has been found on the basis ofexperience to lead to an increased active life of the catalyst(operating life). This also increases the temperature interval in whichthe conversion can be increased by increasing the temperature.

TABLE 2 Dode- C16+- Octenes cenes alkenes T [% by [% by [% by MixtureCat Vol [1] [° C.] weight] weight] weight] C[%] Iso I A 1a 40 60 83.713.3 3 70.5 0.95 B 1b 40 60 92.9 5.2 1.9 67.6 1.1 C 1b 40 80 88.7 9.12.2 70.2 1.15 D 1c 20 70 75.3 20.6 4.1 70.4 1.68 E 1a 40 90 82.4 13.83.8 68.1 1.01 F 1c 20 80 74.8 20.7 4.5 70 1.68 Mixture Mixture havingthe composition shown in table 1 Cat Catalyst used Vol Catalyst volume TMean reaction temperature in the catalyst zone C16+-alkenes Alkeneshaving 16 or more carbon atoms present C Total butene conversion Iso IIso index of the C8 fraction of the oligomerization product

TABLE 3 Catalyst zone 1 Catalyst zone 2 Catalyst zone 3 Vol T Vol T VolT No. Cat S:Ni [1] [° C.] Cat S:Ni [1] [° C.] Cat S:Ni [1] [° C.] C YSTY Iso I 1 (According to the 1a 0 40 60 1c 1 20 70 — — — — 91.3 14.40.24 1.17 present invention) 2 (Comparison 1) 1b 0.34 40 60 1b 0.34 4080 — — — — 90.3 14.1 0.18 1.12 3 (Comparison 2) 1a 0 40 60 1b 0.34 40 80— — — — 91.2 14.5 0.18 1.01 4 (According to the 1b 0.34 40 60 1b 0.34 4080 1c 1 20 80 97.1 15.2 0.15 1.15 present invention) 5 (According to the1a 0 40 60 1b 0.34 40 80 1c 1 20 80 97.4 15.5 0.16 1.06 presentinvention) 6 (Comparison 3) 1b 0.34 40 60 1b 0.34 40 80 1a 0 40 90 96.915.2 0.13 1.11 7 (Comparison 4) 1a 0 40 60 1b 0.34 40 80 1a 0 40 90 97.215.6 0.13 1.01 Cat Catalyst used S:Ni Molar ratio of sulfur to nickel inthe catalyst Vol Catalyst volume T Mean reaction temperature in thecatalyst zone C Total butene conversion Y Yield of alkenes having 8 or12 carbon atoms STY Mean space-time yield, expressed in kilograms ofalkenes having 8 or 12 carbon atoms per liter of catalyst and hour Iso IIso index of the C8 fraction of the oligomerization product

1. A process for oligomerizing an alkene stream over a solid catalystcomprising sulfur and nickel, wherein the oligomerization is carried outin two or more successive catalyst zones and the molar ratio of sulfurto nickel in the first catalyst zone is less than 0.5 and that in thelast catalyst zone is 0.5 or more and, in the case of further catalystzones between the first and last catalyst zones, the molar ratio ofsulfur to nickel in each catalyst zone is not less than that in thepreceding catalyst zone, based on the main flow direction of the feedstream.
 2. The process according to claim 1, wherein the molar ratio ofsulfur to nickel in the first catalyst zone is less than 0.4 and that inthe last catalyst zone is more than 0.6.
 3. The process according toclaim 1, wherein a catalyst obtainable by a process in which aluminumoxide is treated with a nickel compound and a sulfur compound, eithersimultaneously or firstly with the nickel compound and then with thesulfur compound, and the catalyst obtained in this way is subsequentlydried and calcined and a molar ratio of sulfur to nickel of from 0.25:1to 0.38:1 is in this way set in the finished catalyst is used.
 4. Theprocess according to claim 1, wherein a catalyst which consistsessentially of nickel oxide, silicon dioxide, titanium dioxide and/orzirconium dioxide and, if appropriate, aluminum oxide and has a content,after subtraction of the loss on ignition after heating at 900° C., ofnickel oxide, calculated as NiO, of from 10 to 70% by weight, from 5 to30% by weight of titanium dioxide and/or zirconium dioxide, from 0 to20% by weight of aluminum oxide, from 20 to 40% by weight of silicondioxide and from 0.01 to 1% by weight of an alkali metal oxide, with theproviso that the proportions of the individual components add up to100%, and is obtainable by precipitation of an aluminum-free nickel saltsolution or a nickel salt solution containing a dissolved aluminum saltat a pH of from 5 to 9 by addition of this nickel salt solution to analkali metal water glass solution containing solid titanium dioxideand/or zirconium dioxide, drying and heating of the resultingprecipitate at from 350 to 650° C. is used.
 5. The process according toclaim 1, wherein the alkene stream used is a mixture of alkenes andalkanes having from 2 to 6 carbon atoms.
 6. The process according toclaim 1, wherein the alkene stream used is a mixture of butenes andbutanes.
 7. The process according to claim 1, wherein the alkenes of thealkene stream are reacted to an extent of from 65 to 99% in the firstcatalyst zone and the alkenes remaining unreacted after this firstcatalyst zone are reacted to an extent of from 10 to 99% in theremaining catalyst zones.
 8. The process according to claim 2, wherein acatalyst obtainable by a process in which aluminum oxide is treated witha nickel compound and a sulfur compound, either simultaneously orfirstly with the nickel compound and then with the sulfur compound, andthe catalyst obtained in this way is subsequently dried and calcined anda molar ratio of sulfur to nickel of from 0.25:1 to 0.38:1 is in thisway set in the finished catalyst is used.
 9. The process according toclaim 2, wherein a catalyst which consists essentially of nickel oxide,silicon dioxide, titanium dioxide and/or zirconium dioxide and, ifappropriate, aluminum oxide and has a content, after subtraction of theloss on ignition after heating at 900° C., of nickel oxide, calculatedas NiO, of from 10 to 70% by weight, from 5 to 30% by weight of titaniumdioxide and/or zirconium dioxide, from 0 to 20% by weight of aluminumoxide, from 20 to 40% by weight of silicon dioxide and from 0.01 to 1%by weight of an alkali metal oxide, with the proviso that theproportions of the individual components add up to 100%, and isobtainable by precipitation of an aluminum-free nickel salt solution ora nickel salt solution containing a dissolved aluminum salt at a pH offrom 5 to 9 by addition of this nickel salt solution to an alkali metalwater glass solution containing solid titanium dioxide and/or zirconiumdioxide, drying and heating of the resulting precipitate at from 350 to650° C. is used.
 10. The process according to claim 9, wherein thealkene stream used is a mixture of alkenes and alkanes having from 2 to6 carbon atoms.
 11. The process according to claim 10, wherein thealkene stream used is a mixture of butenes and butanes.
 12. The processaccording to claim 11, wherein the alkenes of the alkene stream arereacted to an extent of from 65 to 99% in the first catalyst zone andthe alkenes remaining unreacted after this first catalyst zone arereacted to an extent of from 10 to 99% in the remaining catalyst zones.13. The process according to claim 1, wherein the process is carried outover a fixed catalyst bed in which the molar ratio of sulfur to nickelin the first catalyst zone is less than 0.4 and that in the lastcatalyst zone is more than 0.8.
 14. The process according to claim 2,wherein the process is carried out over a fixed catalyst bed in whichthe molar ratio of sulfur to nickel in the first catalyst zone is lessthan 0.4 and that in the last catalyst zone is more than
 1. 15. Theprocess according to claim 1, wherein the alkenes of the alkene streamare reacted to an extent of from 80 to 99% in the first catalyst zoneand the alkenes remaining unreacted after this first catalyst zone arereacted to an extent of from 10 to 99% in the remaining catalyst zones.16. The process according to claim 1, wherein the alkenes of the alkenestream are reacted to an extent of from 90 to 99% in the first catalystzone and the alkenes remaining unreacted after this first catalyst zoneare reacted to an extent of from 50 to 99% in the remaining catalystzones.